Single reel data tape cartridges, one generation of which is known as 3480 type cartridges, include a reel containing magnetic tape with a leader block attached to the free end of the tape as it extends from the reel. The cartridge is generally rectangular except for one corner which is angled and includes a leader block window. The leader block window holds the leader block and permits the tape to exit from the cartridge for threading through a tape drive when the leader block is removed. When the leader block is snapped into the window, the window is covered.
The leader block is generally rectangular and has a cutout which combines with a pin to hold the tape. The front surface of the leader block includes a slot for engaging an automatic threading apparatus in a reel-to-reel magnetic tape drive apparatus. The front surface of the leader block is generally cylindrical and has a rounded boss which engages an inclined surface of the corner of the cartridge. The rear surface is rounded to form a portion of an arc of a radius to match the periphery of the take-up reel hub in the tape drive apparatus as the leader block fits into the slot in the hub. A compliant section may be formed on the leader block to permit the leader block to compress at its rear surface to accommodate especially the first layer of tape when the tape is wound onto the take-up reel.
The tape drive apparatus includes a threading apparatus which connects to a single reel tape cartridge. The drive apparatus includes a drive motor for the supply reel mounted beneath a base plate so that its shaft extends normal to and slightly above a surface of the base plate. A take-up reel is mounted on the base plate and is attached to the motor. The cartridge reel, when coupled to the motor, lies in substantially the same plane as a hub of the take-up reel so that the tape moves in a plane normal to both motor shafts when being transported. After the threading of the tape, transfer of the tape between the supply reel and the take-up reel is achieved by controlling the driving motors for the reels. The tape passes a roller guide, an air bearing guide, a magnetic transducer head, a second air bearing guide, and a roller of a tension transducer.
The tape drive apparatus also includes a mechanism which interacts with the tape cartridge to sense whether the tape is write-enabled and can be erased and recorded over or whether the tape is write-protected and can not be erased. Typical erasure preventing devices for these single reel cartridges incorporate a mechanism that senses whether a record opening in a wall of the cassette is blocked or unblocked. When the opening is unblocked, the recording function is disabled to prevent accidental erasure.
One type of erasure preventing device is a cylindrical thumbwheel which is rotatable in an opening in the front wall of the cartridge near the corner opposite the corner that has the leader block window. The thumbwheel has a larger diameter central portion which is exposed through the opening and two smaller diameter end portions which are received and rotatable in bosses in the cartridge housing. The central portion includes a flat section which corresponds in size and shape to the opening in the front wall of the cartridge. When the flat section registers with the opening, the tape is write-protected. When the thumbwheel is rotated so the flat section does not register with the opening, the tape is write-enabled.
The two end portions of the thumbwheel are received in and are rotatable in a thumbwheel holder formed in the housing. The holder includes a boss in the cartridge base which receives one end portion and a boss in the cartridge cover 36' which receives the other end portion. The cover boss is shown in FIGS. 4 and 5. However, the thumbwheel can be misaligned during assembly, sometimes resulting in cartridges that do not function properly. There is a need for a cartridge which can reduce the incidents of misplaced erasure preventing devices.